1. Field of Invention
The present invention relates to a lithographic apparatus, a vibration control method and an initialization method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. The scanning movement as described is, in general, performed using a position device including a plurality of electromagnetic actuators and motors. Prior to the synchronous scanning of the substrate and the patterning device, both the patterning device and the substrate are accelerated to a predefined speed using the positioning device. In general, this is done by exerting a force to a chuck or object to which the patterning device or substrate is mounted. When the synchronous scanning movement is done, i.e. the pattern is irradiated on the target portion of the substrate, the positioning device is used to decelerate (or apply a negative acceleration) the patterning device and substrate.
In order to ensure that the pattern of the patterning device is projected on the appropriate target area on the substrate, accurate positioning of the patterning device with respect to the substrate is desired. A mismatch between the actual projection of the pattern and the target area may cause an overlay error which may result in a malfunctioning of the semiconductor device that is made. The accurate positioning of the patterning device and the substrate is enabled by a control unit that controls the positioning device, based on a position measurement of the patterning device and the substrate using a position measurement system such as an interferometer system or an encoder based measurement system. In order to ensure accurate projection of the pattern on the substrate, it is also desirable that the projection optics, which enable the patterned beam originating from the patterning device to be projected on the substrate, are substantially stationary. It has been noticed that due to the scanning movement of the patterning device in particular, the position of the projection optics (e.g. a projection lens) is actually not sufficiently stable but shows some vibrations. These vibrations of the projection lens may have a negative impact on the overlay performance of the lithographic apparatus and on the imaging accuracy.